Cements from βdicarbonyl polymers

ABSTRACT

The invention provides novel ionomer polymers and cements comprising β-dicarbonyl groups. The invention has particular utility as filling materials for restoring teeth and for cementing inlays and crowns into place in the tooth, providing a base or a lining in a tooth cavity, and as adhesives or sealants. The invention provides ionomer cement systems that achieve increased resistance to water absorption and solubility.

This is a continuation of application Ser. No. 08/029,124 filed Mar. 10,1993, now U.S. Pat. No. 5,378,785 which is a Divsion of Ser. No.07/843,420 filed Feb. 27, 1992, now U.S. Pat. No. 5,227,413.

TECHNICAL FIELD

This invention relates to ionomers, as well as to methods of using suchionomers, and cements formed with such ionomers.

BACKGROUND ART

The materials known as dental ionomer cements have many applications indentistry including use as filling materials for restoring teeth and forcementing inlays and crowns into place in the tooth, providing a base ora lining in a tooth cavity, and as adhesives or sealants. An ionomercement is formed by reacting 1) an ionomer polymer with 2) a reactiveglass. This reaction is typically done in water.

Ionomer polymers traditionally have been copolymers of two or moremonomers. For example, itaconic acid and acrylic acid have often beencopolymerized to form such an ionomer polymer. The polymer is formed byreacting the two monomers together using a free radical polymerizationmechanism. The ionomer polymer is called such because of its inherentacidity resulting from the numerous acid pendant groups.

After the polymer is made it is often dissolved in water for latermixing with the reactive glass. The reactive glass used in the ionomercement is often an ion-leachable glass, such as those based on calciumaluminosilicate glasses, or more recently, borate glasses. For a generaldiscussion, see Prosser et al., Polyelectrolyte Cements, Wilson andProsser, eds., Developments in Ionic Polymers--1, Chapter 5, AppliedScience Publishers (London and New York, 1983). The glass is finelyground into a powder to facilitate mixing with the ionomer polymersolution.

In the setting reaction, the glass powder behaves like a base and reactswith the acidic polyelectrolyte, i.e., ionomer polymer, releasing metalions to form a metal polysalt which acts as the binding matrix. Waterserves as the reaction medium and allows the transport of ions in whatis essentially an ionic reaction. The setting reaction is thereforecharacterized as a chemical cure system that proceeds automatically uponmixing the ionomer polymer and glass powder in the presence of water.The cements set to a gel-like state within a few minutes and rapidlyharden to develop strength. See, e.g., Prosser et al., J. Chem. Tech.Biotechnol., 29, 69-87 (1979).

Chelating agents, such as tartaric acid, have been described as usefulfor modifying the rate of setting, e.g., to provide longer working timesfor the cements. See, e.g., U.S. Pat. Nos. 4,089,830, 4,209,434,4,317,681, 4,374,936, and 4,758,612. Longer working times afford thedentist more time to mix the cement and apply the cement to the tooth.Unfortunately, when working times are lengthened by the usual methods,setting times are generally also lengthened. A longer setting timedecreases the efficiency of the dentist and increases the amount of timethe patient must spend in the dental chair. The role of tartaric acidhas been explained as involving the temporary withholding of cationsfrom crosslinking the polyanion chains (i.e., the ionomer) throughcomplex formation. See generally, Prosser et al., PolyelectrolyteCements, supra at Chapter 5.

Many commercially available glass ionomer cements include such chelatingagents, and as a result are characterized by working times that are onthe order of 1 to 2 minutes, but relatively long setting times, e.g., onthe order of 4 to 15 minutes. During this set time the cement must beprotected from being washed away by moisture from the mouth (e.g.,through the use of cotton pads) but also must not be allowed to dry out.Such conditions can lead to discomfort for the patient as well as theadded burden of having to spend extra time in the dentist's chair. Thuspresent day glass ionomer cements, although beneficial clinically, arequite technique-sensitive, as well as time-consuming for the dentist andpatient.

Recently a photocurable ionomer cement has become availablecommercially. This cement system can provide a long working time and canbe cured on demand by exposure to an appropriate source of radiantenergy. This cement system is described in European Patent ApplicationNo. 0 323 120 and is commercially available as Vitrebond™ Light CureGlass Ionomer Liner/Base (available from 3M Company, St. Paul, Minn.55144).

The final set cement must be both durable against frictional wear andresistant to degradation by an aqueous environment. Unfortunately,current ionomer cement systems frequently do not adequately resistdegradation when exposed to water and may over time be undesirablyeroded away.

SUMMARY OF THE INVENTION

Further adjustability of water miscibility and water absorption would bedesirable in order to provide greater flexibility in the formulation ofglass ionomer cement systems. Such adjustability is also desirable inorder to extend the practical application of such cement systems to usesinvolving greater exposure of the hardened cement to aqueousenvironments (e.g., the exposed margin around crowns, sealant surfaces,or exposed restoratives) than is prudently accomplished using currenttechniques and materials.

The present invention relates to novel ionomer polymers comprisingpendent complexing groups, wherein at least one of the pendentcomplexing groups is a β-dicarbonyl group. These polymers are useful informing dental ionomer cements, dental adhesives or sealants.

The present invention provides, in another aspect, ionomer cementsystems that achieve increased resistance to water absorption andsolubility. These systems are prepared using ionomers which comprise apolymer having sufficient pendent complexing groups, wherein at leastone of the pendent complexing groups on the polymer is a β-dicarbonylgroup, to undergo a setting reaction in the presence of a reactive glasspowder and water.

The present invention provides, in another aspect, ionomer cementsystems that are optionally free-radically or cationically crosslinkableand achieve increased resistance to water absorption and solubility.These systems are prepared using crosslinkable ionomers which comprise apolymer having sufficient pendent complexing groups, wherein at leastone of the pendent complexing groups is a β-dicarbonyl group, to undergoa setting reaction in the presence of a reactive glass powder and water,and sufficient pendent polymerizable groups to enable the resultingmixture to be crosslinked by exposure to radiant energy or by afree-radical or cationic mechanism.

The invention also provides methods for preparing and methods for usingsuch ionomer cement systems.

The ionomer cement system of the present invention comprises (a) anionomer polymer comprising at least one pendent β-dicarbonyl group, and(b) a reactive glass powder. Presently preferred optional ingredients ofthe crosslinkable ionomer system include water (present in a form thatdoes not prematurely begin to set the system), appropriatepolymerization initiators, modifying agents, and copolymerizable andnon-copolymerizable cosolvents. Other optional ingredients includepigments, fillers (e.g., pulverized precious or nonprecious metals,silica, quartz or metal oxides), and the like.

The crosslinkable ionomer cement systems of the present invention can beprepared by combining the crosslinkable ionomer and the reactive glasspowder in the presence of water. As with present day cement systems, thewater serves as a reaction medium allowing the transport of ions betweenthe ionomer and the reactive powder, thereby allowing the acid-basechemical cure "setting" reaction to occur. This setting reaction canalso be termed the "cement reaction" in that it will proceed regardlessof the co-existing free-radical or cationic mechanism.

The cured systems of the present invention have been found to provideincreased resistance to degradation by saliva and other aqueousenvironments. As a result, the cement surfaces can be exposed to aqueousenvironments without the need for a separate layer of varnish, glaze, ordental restorative. This feature is particularly beneficial near themargins of crowns and bridges since it is often not practical to varnishor cover these surfaces. While previous ionomer cements have beensusceptible to erosion at these margin surfaces to an undesirabledegree, the present invention substantially resists such erosion and hasgreater longevity. Moreover, by the use of fluoride-containing reactiveglass powders, as explained more fully below, the present inventionprovides the ability to prepare a dental restorative that is bothcrosslinkable and capable of exhibiting cariostatic fluoride release.Such a combination of properties is highly desirable.

The present invention relates to ionomer cement systems useful, forinstance, for the preparation of dental and medical adhesives, bases,liners, luting agents, sealants, and filling materials for restorativeand/or endodontic use.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The term "crosslinkable ionomer", as used herein, refers to a polymerhaving sufficient pendent complexing groups to undergo a settingreaction in the presence of a reactive powder and water, and sufficientpendent polymerizable groups to enable the resulting mixture to bepolymerized, i.e., crosslinked, upon exposure to radiant energy, or viaa redox or cationic reaction.

The terms "reactive powder" or "reactive glass powder", as used herein,refer to a metal oxide or hydroxide, mineral silicate, or ion-leachableglass that is capable of reacting with the ionomer in the presence ofwater to form a hydrogel.

The term "ionomer cement system", as used herein, refers to the unmixed,or mixed but unset and uncured, combination of ionomer, reactive powder,and other optional ingredients, such as water. Such systems include kitsin which the ionomer is employed as a concentrated aqueous solution, formixing directly with the powder, as well as kits in which the ionomer isemployed in a dry blend with the powder, for later mixing with water.Such systems also include kits in which the ionomer is employed as aviscous paste, for mixing directly with a second viscous paste thatcontains as a dispersion the reactive powder.

The term "working time", as used herein, refers to the time between thebeginning of the setting reaction, i.e., when the ionomer and reactivepowder are combined in the presence of water, and the time the settingreaction has proceeded to the point at which it is no longer practicalto perform further physical work upon the system, e.g., spatulate it orreform it, for its intended dental or medical purpose.

The term "setting time", as used herein, refers to the time between thebeginning of the setting reaction in a restoration, and the timesufficient hardening has occurred to allow subsequent clinicalprocedures to be performed on the surface of the restoration. Suchhardening can occur either in the course of the normal setting reactionand/or by crosslinking a polymerizable system.

Non-crosslinkable ionomers of the present invention comprise a polymerhaving sufficient pendent complexing groups to undergo a settingreaction in the presence of a reactive powder and water.

Preferred non-crosslinkable ionomers have the general Formula I:

    B(X).sub.m (Z).sub.p                                       I

wherein

B represents an organic backbone of carbon-carbon bonds, optionallycontaining non-interfering substituents or linking groups such asoxygen, nitrogen or sulfur heteroatoms,

each X independently is a β-dicarbonyl group capable of undergoing asetting reaction in the presence of water and a reactive powder,

each Z independently is an ionic group capable of undergoing a settingreaction in the presence of water and a reactive powder,

m is a number having an average value of 1 or more, and

p is a number having an average value of 0 or more, wherein "m" and "p"represent the average number of pendent X groups and pendent Z groups,respectively, on an average polymer molecule. Preferably m is a numberbetween about 5 and about 500 and p is a number between about 0 andabout 1000. More preferably m is a number between about 10 and about 100and p is a number between about 50 and about 400. Most preferably m is anumber between about 20 and about 40 and p is a number between about 100and about 200.

Preferably the backbone B is an oligomeric or polymeric backbone ofcarbon-carbon bonds, optionally containing non-interfering substituentsor linking groups such as oxygen, nitrogen or sulfur heteroatoms. Theterm "non-interfering" as used herein refers to substituents or linkinggroups that do not unduly interfere with its cement reaction with thereactive powder.

Preferred X groups are β-dicarbonyl groups, with β-ketoester groupsbeing particularly preferred. Particularly preferred/3-ketoester groupsare β-acetoacetyl groups.

Suitable Z groups are oxy acids of phosphorus in oxidation state III orV, phosphonic acid, halophosphonic acid, oxy acids of sulphur inoxidation state VI or IV, and oxy acids of boron in oxidation state III.Preferred Z groups are carboxyl, boric and sulphonic groups withcarboxyl being particularly preferred.

X and Z groups can be linked to the backbone B directly or by means ofany non-interfering organic linking group, such as substituted orunsubstituted alkyl, alkoxyalkyl, aryl, aryloxyalkyl, alkoxyaryl,aralkyl, or alkaryl groups. Preferred non-interfering organic linkinggroups contain water-solubilizing groups, with 1-vinyl-2-pyrrolidone("VP") being particularly preferred.

Non-crosslinkable ionomers of Formula I can be prepared according to avariety of synthetic rotates, including, but not limited to: (1)reacting a polymer of the formula B(Z)_(p+i), where "p" and "i" arenumbers which combined represent the total average number of Z groups onthe starting polymer, with a suitable compound in order to form mpendent X groups either through i Z groups thereby leaving p unreacted Zgroups, or at other positions, and (2) copolymerizing appropriatemonomers, e.g., a monomer containing one or more pendent X groups and amonomer containing one or more pendent Z groups. The second syntheticroute referred to above is preferred.

Synthetic route 1, above, can presently be carried out by the use of a"coupling compound", i.e., a compound containing both a pendent group(i.e., an X group) and a reactive group capable of reacting with thepolymer through a Z group or through a non-interfering group in order toform a covalent bond between the coupling compound and the Z group orthe non-interfering group, thereby linking the pendent group to thebackbone B. Suitable coupling compounds are organic compounds,optionally containing non-interfering substituents and/ornon-interfering linking groups between the pendent group and thereactive group.

Crosslinkable ionomers of the present invention comprise a polymerhaving sufficient pendent complexing groups to undergo a settingreaction in the presence of a reactive powder and water, and sufficientpendent polymerizable groups to enable the resulting mixture to becrosslinked by exposure to radiant energy or by a free-radical orcationic mechanism.

Preferred crosslinkable ionomers have the general Formula II:

    B(X).sub.m (Y).sub.n (Z).sub.p                             II

wherein

B represents an organic backbone,

each X independently is a β-dicarbonyl group capable of undergoing asetting reaction in the presence of water and a reactive powder,

each Y independently is a crosslinkable group capable of undergoing afree-radical or cationic crosslinking reaction,

each Z independently is an ionic group capable of undergoing a settingreaction in the presence of water and a reactive powder,

m is a number having an average value of 1 or more,

n is a number having an average value of 1 or more, and

is a number having an average value of 0 or more, wherein "m", "n" and"p" represent the average number of pendent X groups, pendent Y groupsand pendent Z groups, respectively, on an average polymer molecule.Preferably m is a number between about 5 and about 500, n is a numberbetween about 1 and about 500 and p is a number between about 0 andabout 1000. More preferably m is a number between about 10 and about100, n is a number between about 10 and about 100 and p is a numberbetween about 50 and about 400. Most preferably m is a number betweenabout 20 and about 40, n is a number between about 20 and about 40 and pis a number between about 100 and about 200.

Preferably the backbone B is an oligomeric or polymeric backbone ofcarbon-carbon bonds, optionally containing non-interfering substituentsor linking groups such as oxygen, nitrogen or sulfur heteroatoms. Theterm "non-interfering" as used herein refers to substituents or linkinggroups that do not unduly interfere with either the flee-radical orcationic polymerization reaction of the crosslinkable ionomer or itscement reaction with the reactive powder.

Preferred X groups are β-ketoester groups. Particularly preferredβ-ketoester groups are 2-acetoacetyl groups.

Suitable Y groups include, but are not limited to, polymerizableethylenically unsaturated groups and polymerizable epoxy groups.Ethylenically unsaturated groups are preferred, especially those thatcan be polymerized by means of a free-radical mechanism, examples ofwhich are substituted and unsubstituted acrylates, methacrylates,alkenes and acrylamides. In aqueous systems, polymerizable groups thatare polymerized by a cationic mechanism, e.g., polymerizableethylenically unsaturated groups such as vinyl ether groups andpolymerizable epoxy groups, are less preferred since a free-radicalmechanism is typically easier to employ in such systems than a cationicmechanism.

Suitable Z groups are oxy acids of phosphorus in oxidation state III orV, phosphonic acid, halophosphonic acid, oxy acids of sulphur inoxidation state VI or IV, and oxy acids of boron in oxidation state III.Preferred Z groups are carboxyl, boric and sulphonic groups withcarboxyl being particularly preferred.

X, Y, and Z groups can be linked to the backbone B directly or by meansof any non-interfering organic linking group, such as substituted orunsubstituted alkyl, alkoxyalkyl, aryl, aryloxyalkyl, alkoxyaryl,aralkyl, or alkaryl groups. Preferred non-interfering organic linkinggroups contain water-solubilizing groups, with 1-vinyl-2-pyrrolidone("VP") being particularly preferred.

Crosslinkable ionomers of Formula II can be prepared according to avariety of synthetic routes, including, but not limited to: (1) reactinga polymer of the formula B(Z)_(p+i+j), where "p", "i", and "j" arenumbers which combined represent the total average number of Z groups onthe starting polymer, with a suitable compound in order to form mpendent X groups either through i Z groups, thereby leaving p+j Z groupsunreacted with an X group, or at other positions and with a suitablecompound in order to form n pendent Y groups either through j Z groups,thereby leaving p+i Z groups unreacted with a Y group, or at otherpositions; (2) reacting a polymer of the formula B(X)_(m) (Z)_(p+j),where "m" is a number representing the average number of X groups on thestarting polymer and "p" and "j" are numbers which combined representthe total average number of Z groups on the starting polymer, in orderto form n pendent Y groups either through j Z groups, thereby leaving pZ groups unreacted with a Y group, or at other positions; and (3)copolymerizing appropriate monomers, e.g., a monomer containing one ormore pendent X groups, a monomer containing one or more pendent Y groupsand a monomer containing zero or more pendent Z groups.

The first and second synthetic routes referred to above are presentlypreferred, i.e., the reaction of a polymer with a suitable compound orcompounds to form pendent Y groups, as in route (2), or pendent X and Ygroups, as in route (1).

Synthetic route 1, above, can presently be carried out by the use of a"coupling compound", i.e., a compound containing both a pendent group(i.e., either an X or a Y group) and a reactive group capable ofreacting with the polymer through a Z group or through a non-interferinggroup in order to form a covalent bond between the coupling compound andthe Z group or the non-interfering group, thereby linking the pendentgroup to the backbone B. Suitable coupling compounds are organiccompounds, optionally containing non-interfering substituents and/ornon-interfering linking groups between the pendent group and thereactive group.

The second synthetic route referred to above is presently mostpreferred, i.e., the reaction of suitable compounds with n pendent Ygroups to a polymer of the formula B(X)_(m) (Z)_(p+j) at positions otherthan the X groups. Such groups can be reacted by the use of a "couplingcompound", i.e., a compound containing both a Y group and a reactivegroup capable of reacting with the polymer through a non-interferinggroup, thereby linking the Y group to the backbone B in a pendentfashion. Suitable coupling compounds are organic compounds, optionallycontaining non-interfering substituents and/or non-interfering linkinggroups between the Y group and the reactive group.

Particularly preferred polymerizable ionomers of Formula II are those inwhich each X is a β-ketoester group and each Y is an ethylenicallyunsaturated group that can be polymerized by a free-radical mechanism.Such ionomers are conveniently prepared by reacting a polymer backbone(e.g., a polymer of formula B(X)_(m) (Z)_(p+j) wherein each X is aβ-ketoester group) with a coupling compound containing both anethylenically unsaturated group and a group capable of reacting with acarboxylic acid group or a noninterfering group.

Polymers of formula B(X)_(m) (Z)_(p+j) or B(Z)_(p+i+j) can be preparedby copolymerizing an appropriate mixture of monomers or comonomers.Preferably, such polymers are prepared by free-radical polymerization,e.g., in solution, in an emulsion, or interfacially. Such polymers canbe reacted with coupling compounds in the presence of appropriatecatalysts, as described more fully in the examples below.

Suitable β-dicarbonyl groups for use in preparing ionomers of thepresent invention include:

(1) β-diesters having the formula: ##STR1## wherein: R₁ and R₂ areindependently selected from the group consisting of alkyl, preferablycontaining from 1 to 8 carbon atoms, such as methyl, ethyl, isopropyl,butyl, including substituted alkyl, such as chloromethyl and the like;aryl, preferably containing from 6 to 10 carbon atoms, such as phenyl,including substituted aryl, such as bromophenyl and the like;carbocyclic groups such as cycloaliphatic, such as cyclohexyl andheterocyclic groups, such as pyridyl, and the like; and ethylenicallyunsaturated groups, such as vinyl, allyl, vinylbenzyl, acrylate,acrylamide, methacrylate, methacrylamide, acryloxyalkyl,methacryloxyalkyl, acrylamidoalkyl, and methacrylamidoalkyl; and whereinat least one of R₁ and R₂ is a reactive group capable of reacting withthe polymer thereby linking the pendent group to the backbone;

(2) β-diketones having the formula: ##STR2## wherein R₁ and R₂ are asdescribed above, and;

(3) β-ketoesters having the formula: ##STR3## wherein R₁ and R₂ are asdescribed above.

Coupling compounds suitable for use for preparing the preferred ionomersof the present invention include compounds that contain at least onegroup capable of reacting with Z, or with a noninterfering group, inorder to form a covalent bond, as well as at least one polymerizableethylenically unsaturated group or a β-dicarbonyl group. When Z iscarboxyl, a number of groups are capable of reacting with Z, includingboth electrophilic and nucleophilic groups. Examples of such groupsinclude the following moieties, and groups containing these moieties:--OH, --NH₂, --NCO, --COCl, and ##STR4## When the attaching site is analcohol, a number of groups are capable of reacting with the alcohol.Examples of such groups include the following moieties, and groupscontaining these moieties: --NCO, --COCl, ##STR5##

Examples of suitable coupling compounds to attach Y groups include, forexample, acryloyl chloride, methacryloyl chloride, vinyl azalactone,allyl isocyanate, 2-hydroxyethylmethacrylate, 2-aminoethylmethacrylate,and 2-isocyanatoethylmethacrylate. Other examples of suitable couplingcompounds include those described in U.S. Pat. No. 4,035,321, thedisclosure of which is hereby incorporated by reference. Examples ofpreferred coupling compounds include, for example, the followingmethacrylate compounds and their corresponding acrylates: ##STR6## thefollowing allyl compound: ##STR7## as well as the following β-dicarbonylcompounds: ##STR8## wherein q is 1 to 20, R is H or lower alkyl (e.g.,having 1 to 6 carbon atoms), and R₂ is alkyl or alkoxyalkyl andpreferably contains from 1 to 8 carbon atoms.

Particularly preferred coupling compounds are the following methacrylatecompounds and their corresponding acrylates, wherein R and q are asdefined above. ##STR9## Preferred crosslinkable ionomers of Formula IIare prepared by reacting a polymer of formula B(X)_(m) (Z)_(p+j) whereinX is a β-ketoester group with a coupling compound containing a reactivegroup of the formula NCO. The resultant ionomers, e.g., those of FormulaII above wherein the covalent bond between the Z group or anon-interfering group and the reactive group of the coupling compound isan amide linkage or a urethane linkage, provide an optimal combinationof such properties as adhesion to dentin, mechanical strength, workingtime, fluoride release and the like.

The molecular weight of the resultant ionomers of either Formula I or IIis preferably between about 1000 and about 500,000, more preferablybetween about 5,000 and about 100,000, and most preferably between about10,000 and about 40,000. These ionomers are generally water-miscible,but to a lesser extent than the polymer backbone from which they arederived. Hence, the use of cosolvents, as described more fully below, ispreferred in order to enhance the solubility of the ionomers and achievemore concentrated solutions thereof.

Ionomer cements of the present invention may optionally containconventional polyalkenoic acid polymers in addition to the ionomerpolymers mentioned above. Suitable polyalkenoic acids for use inpreparing ionomer cements of this invention include those homopolymersand copolymers of unsaturated mono-, di-, or tricarboxylic acidscommonly used to prepare glass ionomer cements. Representativepolyalkenoic acids are described, for example, in U.S. Pat. Nos.3,655,605, 4,016,124, 4,089,830, 4,143,018, 4,342,677, 4,360,605 and4,376,835.

Presently preferred polyalkenoic acids are polymers prepared by thehomopolymerization and copolymerization of unsaturated aliphaticcarboxylic acid monomers, for example acrylic acid, 2-chloroacrylicacid, 3-chloroacrylic acid, 2-bromoacrylic acid, 3-bromoacrylic acid,methacrylic acid, itaconic acid, maleic acid, glutaconic acid, aconiticacid, citraconic acid, mesaconic acid, fumaric acid and tiglic acid.

Suitable monomers that can be copolymerized with the unsaturatedaliphatic carboxylic acids include unsaturated aliphatic compounds suchas acrylamide, acrylonitrile, vinyl chloride, allyl chloride, vinylacetate, and 2-hydroxyethyl methacrylate. Ter- and higher polymers maybe used if desired. The polyalkenoic acid should be surgicallyacceptable, that is, it should be substantially free from unpolymerizedmonomers and other undesirable components.

Particularly preferred polyalkenoic acids also include homopolymers ofpolyacrylic acid, and copolymers of acrylic and itaconic acids, acrylicand maleic acids, methyl vinyl ether and maleic anhydride or maleicacid, ethylene and maleic anhydride or maleic acid, and styrene andmaleic anhydride or maleic acid.

The preferred crosslinkable ionomers of the present invention can beformulated in water, either alone or with the use of adjuvants such ascosolvents described in greater detail below. The preferredconcentration of ionomer in aqueous solution is between about 10 andabout 70 percent by weight, based on the weight of the final aqueoussolution, and more preferably is between about 20 and about 50 percentby weight. For optimal use in preparing a cement of the presentinvention, the preferred viscosity of the ionomer solution is betweenabout 60 and about 2000 centistokes, and most preferably between about300 and about 1500 centistokes. Ionomer solutions having higherviscosities will generally be more difficult to mix, and solutions oflower molecular weight ionomer will generally provide cements havinglower strength.

In order to prepare a crosslinkable ionomer cement from the cementsystem of this invention, a crosslinkable ionomer is mixed with areactive powder in the presence of water. Alternatively, thecrosslinkable ionomer is mixed with a reactive powder in the presence ofwater and an acid. Acids for use in the present invention can beinorganic or organic acids, and if organic can be monomeric, oligomericor polymeric. If desired, a precursor to the acid such as acidanhydride, acid halide (including inorganic acid halides such as Lewisacids and organic acid halides), or ester can be used in place of theacid itself, e.g., to generate the desired acid in situ. Suitable acidsinclude mineral acids, carboxylic acids, sulfonic acids, and phenols,with carboxylic acids, alkylsulfonic acids, and arylsulfonic acids beingpreferred. Optionally, and preferably, the cement system also includes apolymerization initiator, thereby providing the ability to achieve ashorter cure time when preparing the resultant cement.

Reactive powders suitable for use in the cement systems of thisinvention include those that are commonly used with ionomers to formionomer cements.

Suitable reactive powders include glasses such as:

(I) SiO₂ --Al₂ O₃ --CaO and

(II) SiO₂ --Al₂ O₃ --CaF₂.

More complex glasses, e.g., four or more component glasses, may also beutilized. Examples of suitable reactive powders are described in theProsser et at. article cited above, the disclosure of which is herebyincorporated by reference, as well as metal oxides such as zinc oxideand magnesium oxide, and ion-leachable glasses, e.g., as described inU.S. Pat. Nos. 3,655,605, 3,814,717, 4,143,018, 4,209,434, 4,360,605 and4,376,835.

Particularly preferred reactive powders for use in the cement systems ofthis invention are those that contain leachable fluoride, since thesustained release of fluoride ions as a byproduct of the settingreactions provides cariostatic benefits. Examples of preferred powdersinclude fluoroaluminosilicate and fluoroaluminoborate ion-leachableglasses.

The crosslinkable ionomer cement systems of the invention can frequentlybe polymerized without the use of one or more polymerization initiators,e.g., by the use of thermal energy or by exposure to a high energypulsed xenon source. Optionally, and preferably, the ionomer cementsystem contains one or more suitable polymerization initiators that actas a source of free-radicals when activated. Such initiators can be usedalone or in combination with one or more accelerators and/orsensitizers.

Polymerization initiators suitable for use in the present inventioninclude electromagnetic radiation-induced polymerization initiators,such as ultraviolet- or visible-light-induced polymerization initiators,that exhibit a desired combination of such properties as stability andefficiency of free-radical production and polymerization initiation.

Examples of suitable ultraviolet-induced polymerization initiatorsinclude, but are not limited to, ketones such as benzil and benzoin, andacyloins and acyloin ethers, commercially available, for example, fromAldrich Chemical Co. Preferred ultraviolet-induced polymerizationinitiators include 2,2-dimethoxy-2-phenylacetophenone ("Irgacure 651")and benzoin methyl ether (2-methoxy-2-phenylacetophenone), bothcommercially available from Ciba-Geigy Corp.

Examples of suitable visible-light-induced initiators include, but arenot limited to, diaryliodonium salts and triarylsulfonium salts, as wellas chromophore substituted halomethyl-s-triazines, such as thosedescribed in U.S. Pat. No. 3,954,475, and halomethyl oxadiazoles such asthose described in U.S. Pat. No. 4,212,970. Such initiators can be usedalone or in combination with suitable. accelerators, e.g., amines,peroxides, and phosphorus compounds, and/or with suitablephotosensitizers, e.g., ketone or alpha-diketone compounds such as,e.g., camphorquinone.

For crosslinkable ionomers that are polymerized by a cationic mechanism,suitable initiators include salts that are capable of generating cationssuch as the diaryliodonium, triarylsulfonium and aryldiazonium salts.

Preferred visible light-induced polymerization initiator systems includesuitable combinations of a diketone, e.g., camphorquinone, and adiaryliodonium salt, e.g., diphenyliodonium chloride, bromide, iodide orhexafluorophosphate, with or without additional hydrogen donors, oraccelerators, such as sodium benzene sulfinate, amines or aminealcohols.

Polymerization initiator, when employed, is preferably present in theionomer cement system in an amount sufficient to achieve the desiredextent of polymerization. Such amount is dependent in part on theextinction coefficient of the initiator and the thickness of the layerto be exposed to radiant energy. Typically, an ultraviolet-inducedpolymerization initiator will be present at about 0.01% to about 5%,based on the weight of the ionomer(s) present, and the components of avisible light-induced polymerization initiator system will generally bepresent at a combined weight of about 0.01 to 5%, and preferably fromabout 0.1 to 5%, based on the weight of the ionomer(s) present.

Polymerization initiators suitable for use in the present inventioninclude redox initiators that exhibit a desired combination of suchproperties as stability and efficiency of free-radical production andpolymerization initiation and water miscibility.

The water-soluble reducing agent and water-soluble oxidizing agent aremost conveniently discussed together. They should react with orotherwise cooperate with one another to produce free-radicals capable ofcrosslinking the ethylenically-unsaturated moiety. The reducing agentand oxidizing agent preferably are sufficiently shelf-stable and free ofundesirable colorization to permit their storage and use under typicaldental conditions. They should be sufficiently water-soluble to permitready dissolution in (and discourage separation from) the othercomponents of the cement. The reducing agent and oxidizing agent shouldalso be sufficiently soluble and present in an amount sufficient topermit an adequate free-radical reaction rate. This can be evaluated bycombining all of the ingredients of the cement except for the fillerunder safelight conditions, and observing whether or not a hardened massis obtained.

Preferred reducing agents include ascorbic acid, cobalt (II) chloride,ferrous chloride, ferrous sulfate, hydrazine, hydroxylamine (dependingupon the choice of oxidizing agent), oxalic acid, thiourea, and salts ofa dithionite or sulfite anion. Preferred oxidizing agents includepotassium persulfate, cobalt (III) chloride, tert-butyl hydroperoxide,ferric chloride, hydroxylamine (depending upon the choice of reducingagent), perboric acid and its salts, and salts of a permanganate orpersulfate anion with potassium persulfate presently preferred. Hydrogenperoxide can also be used, although it has been found to interfere withthe photoinitiator in some instances.

The amount of reducing agent and oxidizing agent should be sufficient toprovide the desired degree of crosslinking of theethylenically-unsaturated component. The preferred amount for each ofthe reducing agent and oxidizing agent is about 0.01 to about 10%, morepreferably about 0.02 to about 5%, based on the total weight (includingwater) of the unset cement components.

The components of the ionomer cement system can be combined, e.g.,blended or mixed, in a variety of manners and amounts in order to formthe crosslinkable ionomer cement of this invention. Suitable combiningtechniques include those commonly employed to mix ionomer cementsystems. In one suitable technique, a concentrated aqueous solution ofionomer is mixed with reactive powder at the time of use. The resultantcombination of ionomer, powder and water allows the setting reaction tobegin. In an alternative technique, the ionomer and powder are providedas a powdered blend under substantially anhydrous conditions, i.e.,conditions in which there is not sufficient water to allow the settingreaction to proceed. Such systems can then be combined with water at thetime of use in order to begin the setting reaction. In a secondalternative technique, the ionomer is employed as a viscous paste, formixing directly with a second viscous paste that contains as adispersion the reactive powder.

The ratio of powder (i.e., reactive powder or powdered blend of ionomerand reactive powder) to liquid in such techniques is an important factorin determining the workability of the mixed ionomer cement systems.Ratios higher than about twenty to one (powder to liquid, by weight)tend to exhibit poor workability, while ratios below about one to one(powder to liquid, by weight) tend to exhibit poor mechanicalproperties, e.g., strength, and hence are not preferred. Preferredratios of powder to liquid by weight are on the order of about one toone to about five to one.

If desired, the cements of the invention can contain adjuvants such aspigments, nonvitreous fillers, inhibitors, accelerators, viscositymodifiers, surfactants, and other ingredients that will be apparent tothose skilled in the art. Optional other ingredients, such aspolymerization initiators, modifying agents and cosolvents can be addedat any time and in any manner that does not prematurely begin thesetting reaction or the crosslinking reaction. Modifying agents andchelating agents can be used in the ionomer cement systems of thepresent invention in order to provide prolonged working times. Examplesof suitable modifying agents are described in applicant's copending U.S.patent application Ser. No. 07/605,749, the disclosure of which ishereby incorporated by reference. Examples of suitable chelating agents,such as tartaric acid, are described in U.S. Pat. Nos. 4,089,830,4,209,434, 4,317,681, 4,374,936, and 4,758,612.

Cosolvents useful in the present invention include, but are not limitedto, low molecular weight organic solvents. The word "cosolvent", as usedherein refers to a material that aids in the dissolution of an ionomerin water, in order to form a homogeneous aqueous solution of cosolventand ionomer. Suitable cosolvents include non-copolymerizable organicsolvents and copolymerizable low molecular weight hydrophilic alkenylsolvents. The word "copolymerizable" as used herein refers to theability of the cosolvent to cure compatibly with the ionomers used inthe invention. Copolymerizable cosolvents can be added to the ionomercement systems of this invention for a variety of reasons, for instance,to provide a homogeneous solution of a crosslinkable ionomer havinginherently low aqueous solubility, to shorten the exposure of radiantenergy needed to cure the system, or to vary the physical properties,e.g., the flexibility, of the resultant cured ionomer cement. Examplesof suitable cosolvents include non-copolymerizable cosolvents such asethanol, propanol, and glycerol, and copolymerizable cosolvents such as2-hydroxylethylmethacrylate or 2-hydroxypropylmethacrylate.

Sufficient amounts of each component in the cement systems of thepresent invention should be employed to obtain the desired working time.Preferably such systems will provide a working time of at least aboutone minute and less than about 30 minutes and most preferably greaterthan two minutes and less than about 20 minutes, during which time thesystems can be cured by exposure to an appropriate source of radiantenergy or by a free-radical or cationic mechanism. For the sake ofbrevity this discussion will focus on dental applications, andparticularly, the curing of such systems in situ, e.g., in the mouth ofa patient.

One method of curing the ionomer cement system is accomplished byexposure to any source of radiant energy capable of causing the desiredextent of polymerization of the crosslinkable ionomer. Suitable radiantenergy sources afford a desired combination of such properties assafety, controllability, suitable intensity, and suitable distributionof incident energy. For a general discussion, see "Radiation Curing",Kirk-Othmer Encyclopedia of Chemical Technology, 3d Ed., Vol. 19, pp.607-624 (1982). Preferred radiant energy sources are ultraviolet orvisible light sources whose emission spectra correspond closely with theabsorption range of the polymerization initiator in the ionomer cementsystem. For instance, sources emitting ultraviolet light at wavelengthsbetween about 335 and 385 nm, and sources emitting visible light in theblue region at wavelengths between about 420 and 480 nm are preferredfor use with the preferred ultraviolet- and visible-light-inducedpolymerization initiators, respectively. For polymerizing cement systemsin the mouth, visible light radiation such as that provided by standarddental curing lights is particularly preferred.

Upon exposure of an ionomer cement system of the present invention to anappropriate source of radiant energy, the system rapidly begins to cure,e.g., within about 45 seconds, and preferably within about 30 seconds.The restoration generally exhibits the greatest degree of cure at itssurface, where the radiant energy is most intense. The surface of therestoration therefore can be crosslinked sufficiently to allowsubsequent procedures to be performed on the restoration, while theinterior of the restoration is allowed to harden fully by means of theongoing setting reaction or by an ongoing redox reaction. Thus, if theradiant energy exposure step is omitted, the usual setting and redoxcrosslinking will occur, ultimately resulting in the hardening of thematerial, even in the dark. This phenomenon offers a unique advantage inthat a relatively deep restoration can be prepared by rapidlycrosslinking the outer surface of the restoration instantly by exposureto radiant energy, allowing the inner portions of the restoration tocure more slowly by the usual setting reaction and/or the ongoing redoxreaction. As a result, the dentist can continue to carry out furtherrestorative procedures, e.g., layering further ionomer cement on thehardened surface, while the inner portions continue to harden. This canresult in a substantial saving of time for the practitioner and patient.

The ionomer cements of this invention can be used in a variety ofapplications in the dental or medical fields in which a bulk curablematerial of low shrinkage, increased resistance to degradation by water,and that will adhere well to the surrounding tooth or bone structure isdesired. For instance, these cements can be used as dental restorativesfor lining or basing Class I, II, III and V restorations, forcementation, as sealants, and as bulk filling materials.

The present invention will be further understood in view of thefollowing examples which are merely illustrative and not meant to limitthe scope of the invention. Unless otherwise indicated, all parts andpercentages are by weight.

EXAMPLE 1

Synthesis of Non-crosslinkable Ionomer Containing β-Ketoester andPendent Carboxyl Groups

A non-crosslinkable ionomer was prepared from the ingredients set outbelow in TABLE I:

                  TABLE I    ______________________________________    Ingredient (in parts)                    Example No. 1    ______________________________________    Acrylic acid    5.76    Itaconic acid   2.60    AAEM.sup.1      4.28    AIBN.sup.2      0.09    THF.sup.3       66.40    ______________________________________     .sup.1 "AAEM" = 2Acetoacetoxyethyl methacrylate.     .sup.2 "AIBN" = Azobisisobutyronitrile.     .sup.3 "THF" = Tetrahydrofuran.

The itaconic acid was dissolved in THF in a glass reactor. Acrylic acid,AAEM and AIBN were then added sequentially to give a clear solution. Thereaction vessel was fitted with nitrogen inlet tube, thermometer andreflux condenser. The reaction mixture was flushed with nitrogen for 15minutes and then heated for 4 hours at 65°-70° C. A further portion ofAIBN (0.09 parts) was then added to stop the reaction and the heat wasturned off. A portion of the THF was evaporated which caused part of thepolymer to separate out. The reaction mixture was precipitated in 10volumes of ethyl acetate, filtered, washed and then dried in vacuo.

EXAMPLES 2-3

Synthesis of Crosslinkable Ionomers Containing β-Ketoester and PendentHydroxy Groups

Two crosslinkable ionomers were prepared from the ingredients set outbelow in TABLE II:

                  TABLE II    ______________________________________    Ingredient      Example No.    (in parts)      2       3    ______________________________________    STEP I    AAEM            1.6     1.6    GM.sup.1        2.14    2.14    THF             17.8    4.45    AIBN            0.013   0.013    STEP II    DBTDL.sup.2     0.206   0.067    BHT.sup.3       0.01    0.01    THF             17.8    6.67    IEM.sup.4       0.31    0.62    THF             4.44    2.22    ______________________________________     .sup.1 "GM" = 2Glyceryl methacrylate.     .sup.2 "DBTDL" = Dibutyltin dilaurate.     .sup.3 "BHT" = Butylated hydroxytoluene.     .sup.4 "IEM" = 2Isocyanatoethyl methacrylate.

For STEP I of each Example, the AIBN was added to a solution of AAEM andGM in THF. Each reaction mixture was flushed with nitrogen for 15-20minutes and then heated at 60° C. under nitrogen atmosphere for 18hours. In STEP II, to each resultant viscous mixture were added DBTDLand BHT in THF, followed by the addition of IEM dissolved in THF. An airbleed was introduced and each mixture heated at 45° C. for 14 hours.Each reaction mixture was precipitated in ethyl acetate and dried invacuo. Infrared spectral analysis confirmed the presence of ethylenicunsaturation, β-ketoester groups and pendent hydroxy groups.

EXAMPLE 4

Synthesis of Crosslinkable Ionomer Containing β-Ketoester and PendentCarboxyl Groups

A crosslinkable ionomer was prepared from the ingredients set out belowin TABLE III:

                  TABLE III    ______________________________________    Ingredient (in parts)                    Example No. 4    ______________________________________    Acrylic acid    5.76    Itaconic acid   2.60    AAEM            4.28    AIBN            0.09    THF             66.4    IEM             3.1    ______________________________________

The itaconic acid was dissolved in THF in a glass reactor. Acrylic acid,AAEM and AIBN were then added sequentially to give a clear solution. Thereaction vessel was fitted with nitrogen inlet tube, thermometer andreflux condenser. The reaction mixture was flushed with nitrogen for 15minutes and then heated for 4 hours at 65°-70° C. The reaction mixturewas cooled to 35°-40° C. and the nitrogen inlet was replaced with an airinlet tube. BHT (0.05 parts) and DBTDL (1.05 parts) were added, followedby the dropwise addition of IEM dissolved in 4.2 parts of THF. Thereaction was allowed to proceed for a further period of 1 hour. Thepolymer was precipitated in ten volumes of ethyl acetate, filtered,washed with more ethyl acetate and then dried.

EXAMPLES 5-7

Synthesis of Crosslinkable Ionomers Containing β-Ketoester And PendentPyrrolidone Groups

Three crosslinkable ionomers were prepared from the ingredients set outbelow in TABLE IV:

                  TABLE IV    ______________________________________    Ingredient Example No.    (in parts) 5            6       7    ______________________________________    AAEM       8.56         12.8    6.42    VP.sup.1   4.44         2.2     3.33    Acrylic Acid               1.45         1.44    2.88    IEM        3.1          3.1     6.2    ______________________________________     .sup.1 "VP" = 1vinyl-2-pyrrolidone.

For each Example, the first three ingredients listed in TABLE IV and0.16 parts AIBN were dissolved in 66.38 parts THF and charged into athree-necked glass reaction vessel fitted with mechanical stirrer,nitrogen inlet tube, addition funnel and thermometer. Each reactionmixture was flushed with nitrogen for 15 minutes and then heated at 60°C. under nitrogen atmosphere for 20 hours. To each resultant viscousmixture were added 0.266 parts DBTDL and 0.01 parts BHT in 22.15 partsTHF. Each reaction mixture was cooled to 40° C. and an air bleedintroduced. This was followed by the slow addition of the amount of IEMlisted in TABLE IV. Each reaction mixture was allowed to stir at 40° C.for an additional 3.5 hours. Each mixture was added to a 10-fold excessof ethyl acetate, followed by filtration, washing, and drying in vacuo.The structure of each reaction product was confirmed by nuclear magneticresonance spectral analysis. The weight average molecular weight ("M_(w)") of the reaction product of Ex. No. 5 and Ex. No. 7 was determined bygel permeation chromatography ("GPC") to be 65,300 and 55,300respectively with a polydispersity of 3.54 and 2.54 respectively.

EXAMPLES 8-13

Preparation of Ionomer Liquids

Ionomer liquids were formulated by mixing together the ingredients setout below in TABLE V:

                  TABLE V    ______________________________________    Ingredient Example No.    (in parts) 8       9        10   11   12    13    ______________________________________    Ionomer.sup.1               0.60    0.51     4.0  5.2  4.5   4.5    HEMA.sup.2 0.94    0.45     --   4.76 --    --    TAMG.sup.3 --      --       --   --   3.5    Water.sup.4               1.40    0.73     2.4  7.14 1.0   5.5    Copoly 4:1 0.61    0.51     --   5.2  3.0   --    (Acrylic:Itaconic)    Acid.sup.5    CPQ.sup.6  0.01     0.0025   0.05                                     0.05  0.03 --    (C.sub.6 H.sub.5).sub.2 I.sup.+ PF.sub.6.sup.-               --      --       --   --    0.012                                                --    BHT        --      --       --   --    0.006    GMA.sup.7  --      --       3.6  --   --    --    ______________________________________     .sup.1 Examples 8-10 were prepared using the ionomers of Examples 2-4     respectively. Examples 11 and 12 were prepared using the ionomer of     Example 6. Example 13 was prepared using the ionomer of Example 1.     .sup.2 "HEMA" = 2Hydroxyethyl methacrylate.     .sup.3 "TAMG" = Tetracrylamidomethyl glycouril.     .sup.4 Distilled water.     .sup.5 Ethylenically unsaturated acidic copolymer prepared like the     precipitated dry polymer of EXAMPLE 11 of European Published Pat.     Application No. 0 323 120.     .sup.6 "CPQ" = Camphoroquinone.     .sup.7 "GMA" = Glyceryl dimethacrylate.

For each Example, the ingredients were stirred at room temperature (25°C.) under safelight conditions until a homogeneous solution wasobtained. The crosslinkable solutions (Ex. No. 8-12) were protected fromexposure to ambient or artificial light by storage in an opaquecontainer.

EXAMPLE 14

Preparation of Ionomer Liquid with Additional Protonic Acid

To 3 parts AAEM/VP polymer of Example 6 was added 17 parts water and 1part ethanol. Hydrochloric acid ("HCI") was added to the solution tobring the pH to 1.5.

EXAMPLES 15-21

Preparation of Surgical Cements

The ingredients set out below in TABLE VI were mixed, melted in an arcfurnace at about 1350°1450° C., poured from the furnace in a thin streamand quenched using chilled rollers to provide an amorphous single-phasefluoroaluminosilicate glass:

                  TABLE VI    ______________________________________    Ingredient    Parts by Weight    ______________________________________    SiO.sub.2     27.00    Al.sub.2 O.sub.3                  0.80    P.sub.2 O.sub.5                  0.94    AlF.sub.3     23.00    Na.sub.2 AlF.sub.6                  10.65    ZnO           21.00    MgO           2.12    SrO           12.55    B.sub.2 O.sub.3                  1.94    ______________________________________

98 Parts of the glass were ball-milled with 2 parts diphenyliodoniumhexafluorophosphate to provide a pulverized frit which was screenedthrough a 44 micron mesh screen. Surface area was measured using theBrunauer, Emmet and Teller (BET) method and determined to be 1.1-1.3 m²/g.

The ionomer liquids of Examples 8-14 were independently mixed with theabove prepared glass at a powder:liquid ratio of 1.4:1.0 to provide thecements of Examples 15-21 respectively. Each mixture was hand spatulatedat about 20° C. for approximately 10 seconds to provide a smooth creamycement. A sample of each of the cement mixtures of Examples 15, 16 and18 was placed on a mixing pad and irradiated with a "VISILUX 2" dentalcuring light (available from 3M Company, St. Paul, Minn. 55144) for 20seconds. A hard, cured cement was obtained that showed no indentationwhen impressed with a 400 g Gilmore needle per ISO specification 7489.Samples of the cement of Example 15 were used to measure adhesion todentin and other samples of each cement were used to measure diametraltensile strength ("DTS") and compressive strength ("CS") of the curedcement.

For measurement of adhesion to dentin, bovine teeth of similar age andappearance were partially embedded in circular acrylic disks so that theenamel was exposed. The exposed portion of each tooth was ground flatand parallel to the acrylic disk using Grade 120 silicon carbidepaper-backed abrasive mounted on a lapidary wheel, until the dentin wasexposed. Further grinding and polishing of the teeth was carried out bymounting Grade 320 and then Grade 600 silicon carbide paper-backedabrasive on the lapidary wheel. During the grinding and polishing steps,the teeth were continuously rinsed with water. The polished teeth werestored in distilled water and used for testing within 2 hours afterpolishing. The polished teeth were removed from the water and driedusing a stream of compressed air.

A 0.250 mm thick and 19.05 mm square tape film with a 5 mm diametercircular hole through the film was placed over the exposed dentin of thepolished tooth. A 1.4:1.0 powder:liquid cement mixture that had beenhand-spatulated at about 20° C. for about 30 seconds was placed in thehole and cured with a VISILUX 2 curing light for 30 seconds. The tapewas removed without disturbing the cement layer and a thin layer of"SCOTCHBOND 2" dental adhesive (3M) was applied to the cement layer. Theadhesive was cured with a VISILUX™ 2 curing light for 20 seconds. A moldmade from a 2.5 mm thick polytetra-fluoroethylene sheet with a 5 mmdiameter circular hole through the sheet was fitted with a sleeve madefrom a no. 4 gelatin capsule and clamped to each polished toothcoaxially with the cured cement. The gelatin capsule sleeve was thenfilled with "P-50" light cure resin bonded ceramic restorative (3M,universal shade) and cured with a VISILUX™ 2 curing light for 30seconds, allowed to stand for about 5 minutes at room temperature, thenstored in distilled water at 37° C. for 24 hours. The molds were thencarefully removed, leaving a molded cement button attached to eachtooth.

Adhesive strength was evaluated by mounting the acrylic disk in a holderclamped in the jaws of an "INSTRON" tensile testing apparatus with thepolished tooth surface oriented parallel to the direction of pull. Aloop of 0.44 mm diameter orthodontic wire was placed around the base ofthe cement button adjacent to the polished tooth surface. The ends ofthe orthodontic wire were clamped in the pulling jaw of the tensiletesting apparatus, placing the bond in shear stress. The bond wasstressed until it (or the cement button) failed, using a crosshead speedof 2 mm/min. The average adhesion value for six specimens for the cementof Example 15 was 6.53 Mpa.

For DTS and CS measurements, the ionomer liquids of Examples 8-14 wereindependently hand spatulated with the glass for one minute at a 1.4:1.0powder:liquid ratio, then packed into a 4 mm inside diameter glass tube,capped with silicone rubber plugs, and axially compressed at about 0.28Mpa. About 1.5 minutes after the start of mixing, the samples ofExamples 15-19 were exposed for 80 seconds to light from twooppositely-disposed VISILUX 2 curing lamps and then the axial pressurewas removed. The samples were allowed to stand for one hour at ambientpressure, 90%+relative humidity and 37° C. The samples were cut on adiamond saw to form cylindrical plugs 2 mm long for measurement of DTSand 8 mm long for measurement of CS. The plugs were stored in distilledwater at approximately 37° C. for about 24 hours. DTS and CS values weredetermined according to ISO specification 7489, using a crosshead speedof 1 mm/min and an average of at least 5 samples.

The DTS and CS of the cements of Examples 15-21 were measured and thevalues are set out below in TABLE VII. As a control, VITREBOND powderand liquid were mixed at a 1.4:1.0 powder:liquid ratio and the DTS andCS measured as described above for Examples 15-19. As a comparison, theDTS and CS, as cited in the literature, of another commerciallyavailable cement, "KETACBOND" (available from ESPE-Premier Sales Co.,Norristown, Pa. 19404);are provided in TABLE VII.

                  TABLE VII    ______________________________________    Ex.       Ionomer Liqauid DTS     CS    No.       of Ex. No.      (MPa)   (MPa)    ______________________________________    15         8              14.3    82.1    16         9              10.5    51.4    17        10              19.4    142.8    18        11              12.4    84.8    19        12              19.3    136.6    20        13              16.6    102.1    21        14              9.1     22.9    VITREBOND                 13.9    90.3    KETACBOND                 5.5     69.0    ______________________________________

EXAMPLE 22

Solubility of Surgical Cements

Two sample disks of the cement of Example 19 were prepared according tothe procedure of ISO 7489. Freshly mixed cement was placed in a mold anda nichrome wire was embedded into each disk. The specimens were exposedto a VISILUX 2 curing light for 60 seconds, followed by storage for 1hour at 37° C. and 90%+relative humidity.

Two commercially available cements, VITREBOND and KETACBOND weresimilarly prepared. The KETACBOND samples were not light cured, but wereput in a 37° C. oven for 1 hour instead.

The prepared samples of each cement were suspended in 50 ml of water ina tared bottle. The bottles were placed in a 37° C. oven for 23 hours.The samples were then removed and the water allowed to evaporate. Thesolubility of each cement was determined according to ISO 7489 and isset out below in TABLE VIII.

                  TABLE VIII    ______________________________________    Cement        Solubility in Water (%)    ______________________________________    Ex. No. 19    0.06    VITREBOND     0.17    KETACBOND     0.30    ______________________________________

The data of TABLE VIII show that a substantial decrease in watersolubility was observed :For a cement of the present invention comparedto the water solubility of two commercially available cements.

Comparative Examples 1-3

A polycarboxylic acid ionomer was prepared from the ingredients set outbelow in TABLE IX:

                  TABLE IX    ______________________________________    Ingredient           Parts    ______________________________________    Acrylic acid         5.7    parts    Itaconic acid        2.6    parts    AIBN                 0.08   parts    THF                  66     parts    ______________________________________

The itaconic acid was dissolved in THF in a glass reactor. Acrylic acidand AIBN were then added sequentially to give a clear solution. Thereaction vessel was fitted with nitrogen inlet tube, thermometer andreflux condenser. The reaction mixture was flushed with nitrogen for 15minutes and then heated for 4 hours at 65°-70° C. The reaction mixturewas allowed to cool down and then precipitated in ten volumes of ethylacetate, filtered, washed and then dried in vacuo.

Three ionomer liquids were formulated by mixing together the ingredientsset out below in TABLE X.

                  TABLE X    ______________________________________    Ingredient    Example No.    (in parts)    C1          C2.sup.1                                     C3    ______________________________________    Ionomer of this                  2.25        2.25   2.25    example    Water         2.75        2.75   2.75    Aluminum      --          0.50   --    acetylacetonate    Ethyl acetoacetate                  --          --     0.50    ______________________________________     .sup.1 This mixture was shaken for four hours at room temperature but the     solid failed to dissolve into the liquid phase.

Surgical cements were prepared and tested as described for Examples 20and 21. The DTS and CS of the cements of Comparative Examples C1 and C3were measured and the values set out in TABLE XI. Examples 20 and 17 arealso listed for comparison.

                  TABLE XI    ______________________________________    Ex. No.       DTS (MPa.) CS (MPa.)    ______________________________________    C1            13.0       68.3    C3            11.3       60.7    20            16.6       102.1    17            19.4       142.8    ______________________________________

I claim:
 1. A dental ionomer cement system, comprising: a polymer havinga weight average molecular weight of at least 1,000 and comprising oneor more pendent β-dicarbonyl groups capable of undergoing a settingreaction in the presence of water and a reactive powder, and one or morepolymerizable groups capable of crosslinking said polymer; and areactive powder.
 2. A dental ionomer cement system according to claim 1,wherein said one or more β-dicarbonyl groups are selected from the groupconsisting of β-ketoester groups, β-diester groups, and β-diketonegroups, and wherein said polymer has a weight average molecular weightbetween 5,000 and 100,000.
 3. A dental ionomer cement system accordingto claim 1, wherein said one or more β-dicarbonyl groups are β-ketoestergroups.
 4. A dental ionomer cement system according to claim 1, whereinsaid polymer further comprises an ionic group which is also capable ofundergoing a setting reaction in the presence of water and a reactivepowder.
 5. A dental ionomer cement system according to claim 4, whereinsaid ionic group is an alkanoic acid group.
 6. A dental ionomer cementsystem according to claim 1, further comprising a polyalkenoic acidwhich is capable of undergoing a setting reaction in the presence ofwater and a reactive powder.
 7. A dental ionomer cement system accordingto claim 1, further comprising water, present in the form of an aqueoussolution of said polymer, and said powder is provided as a separatecomponent for mixing with said solution.
 8. A dental ionomer cementsystem according to claim 1, wherein said polymer and said reactivepowder are each separately provided as viscous pastes for mixing witheach other.
 9. A dental ionomer cement system according to claim 1,further comprising a visible-light-induced polymerization initiatorwhich is capable of initiating the crosslinking reaction.
 10. A methodfor adhering to or coating hard tissue, comprising the steps of:(a)applying to said hard tissue a cement comprising:(i) a polymer having aweight average molecular weight of at least 1,000 and comprising one ormore pendent β-dicarbonyl groups capable of undergoing a settingreaction in the presence of water and a reactive powder and one or morepolymerizable groups capable of crosslinking said polymer; and (ii) areactive powder, and (b) hardening said cement.
 11. A method accordingto claim 10, wherein said one or more β-dicarbonyl groups areβ-ketoester groups.
 12. A method according to claim 10, wherein saidpolymer further comprises an ionic group which is also capable ofundergoing a setting reaction in the presence of water and a reactivepowder.
 13. A method according to claim 10, wherein said cement furthercomprises a polyalkenoic acid which is capable of undergoing a settingreaction in the presence of water and a reactive powder.
 14. A methodaccording to claim 10, wherein said cement further comprises water,present in a form that does not prematurely begin to set the system. 15.A method according to claim 10, wherein said cement further comprisespolymerization initiator which is capable of initiating the crosslinkingreaction.
 16. A method according to claim 14, wherein said polymer has aweight average molecular weight between 5,000 and 100,000.